Ribonucleotide diphosphate reductase (RDPR) catalyzes the reductive conversion of ribonucleotides to 2'deoxyribonucleotides. This is a crucial and probably rate-determining step in the biosynthesis of genetic material, and represents the branching point between RNA and DNA synthesis [1-3]. RDPR is essential for DNA replication and cell division, and the transcription of RDPR genes is under strict cell-cycle regulation. For these reasons, this enzyme is a logical target for the design of anticancer chemotherapeutic agents [2]. A number of DNA viruses (e.g., herpesviruses) encode for their own RDPR, making these viral versions of the enzyme an attractive target for antiviral chemotherapy. Also, combinations of RDPR inhibitors and reverse transcriptase inhibitors results in synergistic inhibitory effects on human immunodeficiency virus type 1 (HIV-1). Hence, RDPR may also be considered as a target for augmenting the activity of reverse transcriptase inhibitors for the treatment of AIDS. Despite the critical role RDPR inhibitors and reverse transcriptase inhibitors results in synergistic inhibitory effects on human immunodeficiency virus type 1 (HIV-1). Hence, RDPR may also be considered as a target for augmenting the activity of reverse transcriptase inhibitors for the treatment of AIDS. Despite the critical role RDPR plays in cell biology and virology, important structural and mechanistic questions remain. The general goal of this project is to develop mechanism-based nucleotide inactivators of RDPR, 2'-C-substituted ribonucleotides 1-5 (Figure 1) via a short, efficient, flexible method developed in this laboratory; (2) to evaluated the inhibitory effects of 1-5 on murine RDPR; (3) to evaluate the inhibitory effects of 1-5 on vaccinia virus and herpes simplex type 1 virus RDPR; (4) to evaluate the antiproliferative effects of the nucleoside counterparts of 1-5 toward several leukemia cell lines; and (5) to evaluate the ability of the nucleoside counterparts of 1-5 to inhibit the replication of a variety of viruses in cell culture assays.